Consolidated Statements of Cash Flows (USD $) In Thousands, unless otherwise specified	12 Months Ended
	Dec. 31, 2014	Dec. 31, 2013	Dec. 31, 2012
CASH FLOWS FROM OPERATING ACTIVITIES:
Net income	$ 21,860	$ 20,046	$ 19,039
Adjustments to reconcile net income to net cash provided by operating activities:
Gain on sales of mortgage loans held for sale	(757)	(1,564)	(297)
Gain on sale of fixed assets	(5)	(1)	(1)
Net gains on sales of securities	(450)	(3,019)	(1,843)
Provision for loan losses	2,050	2,710	4,150
Deferred tax benefit	(3,613)	(2,929)	(2,104)
Net depreciation and amortization	2,937	5,358	6,445
Decrease (increase) in accrued interest receivable	298	(728)	211
Decrease in prepaid FDIC assessments		2,773	1,562
Gain on sales of other real estate owned	(60)		(1)
Increase in other assets	(2,849)	(5,693)	(3,113)
Increase in other liabilities	2,976	4,043	1,070
Net cash provided by operating activities	22,387	20,996	25,118
CASH FLOWS FROM INVESTING ACTIVITIES:
Proceeds from maturities of short-term investments	4,617	22,367	38,397
Purchase of short-term investments	(2,131)	(9,617)	(37,413)
Proceeds from call of Federal Home Loan Bank of Boston stock	680	284	385
Purchase of Federal Home Loan Bank of Boston stock	(7,524)	(3,210)
Proceeds from calls/maturities of securities available-for-sale	153,832	256,420	532,734
Proceeds from sales of securities available-for-sale	40,285	224,045	294,881
Purchase of securities available-for-sale	(176,224)	(543,072)	(998,955)
Proceeds from calls/maturities of securities held-to-maturity	267,486	121,121	88,628
Purchase of securities held-to-maturity	(181,411)	(344,455)	(185,346)
Proceeds from sales of portfolio loans	44,501	93,337	14,457
Net increase in loans	(111,528)	(245,670)	(143,332)
Proceeds from sales of other real estate owned	615		1,584
Proceeds from sales of fixed assets	5	1	1
Capital expenditures	(3,104)	(1,820)	(4,300)
Net cash provided by (used in) investing activities	30,099	(430,269)	(398,279)
CASH FLOWS FROM FINANCING ACTIVITIES:
Net increase (decrease) in time deposit accounts	921	(37,759)	(13,518)
Net increase in demand, savings, money market and NOW deposits	20,831	308,525	334,007
Net proceeds from the exercise of stock options	361	43	304
Cash dividends	(2,196)	(2,191)	(2,186)
Net (decrease) increase in securities sold under agreements to repurchase	(2,080)	23,050	48,070
Net increase (decrease) in other borrowed funds	140,356	60,000	(48,999)
Net cash provided by financing activities	158,193	351,668	317,678
Net (decrease) increase in cash and cash equivalents	210,679	(57,605)	(55,483)
Cash and cash equivalents at beginning of year	94,678	152,283	207,766
Cash and cash equivalents at end of year	305,357	94,678	152,283
Cash paid during the year for:
Cash paid during the period for interest	19,168	18,812	19,597
Cash paid during the period for income taxes	4,493	4,008	3,348
Change in unrealized gains on securities available-for-sale, net of taxes	1,122	(13,375)	4,011
Change in unrealized losses on securities transferred to held-to-maturity, net of taxes	3,188	(13,667)
Pension liability adjustment, net of taxes	(8,318)	5,626	(1,839)
Transfer of loans to other real estate owned	555		400
Transfer of securities available-for-sale to held-to-maturity		$ 987,037